In vitro, antimicrobial activity of dodecyl-di(β- hydroxyethyl)-benzene ammonium chloride (DOBAC) and dodecyl-triphenyl-phosphonium bromide (DTPB), two surface active compounds, was tested in the presence of corticosteroids in doses which have been shown to enhance metabolic activity of bacteria. The experiments were performed with the Warburg technique measuring oxygen consumption of bacterial suspension in resting phase. Staph. aureus and B. pyocyaneus, three strains of each, served as test organisms. On Staph. aureus a marked decrease in antimicrobial activity of DOBAC and DTPB was encountered after addition of corticosteroids (hydrocortisone, methylprednisolone, fluocinolone). A similar, but weaker effect was present in B. pyocyaneus, too. These data suggest that in vitro tests should be performed before different chemotherapeutic are combined in one preparation.